Sailfish habitat utilization and vertical movements in the southern gulf of mexico and florida straits

BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. Sailfish Habitat Utilization and Vertical Movements in the Southern Gulf of Mexico and Florida Straits Author(s): David W. Kerstetter, Shannon M. Bayse, and Jenny L. FentonJohn E. Graves Source: Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science, 3(1):353-365. 2012. Published By: American Fisheries Society URL: http://www.bioone.org/doi/full/10.1080/19425120.2011.623990 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 3:353–365, 2011 C  American Fisheries Society 2011 ISSN: 1942-5120 online DOI: 10.1080/19425120.2011.623990 ARTICLE Sailfish Habitat Utilization and Vertical Movements in the Southern Gulf of Mexico and Florida Straits David W. Kerstetter,* Shannon M. Bayse, and Jenny L. Fenton Nova Southeastern University Oceanographic Center, 8000 North Ocean Drive, Dania Beach, Florida 33004, USA John E. Graves Virginia Institute of Marine Science, College of William and Mary, Route 1208 Greate Road, Gloucester Point, Virginia 23062, USA Abstract Pop-up satellite archival tags (PSATs) were deployed on 19 sailfish Istiophorus platypterus captured in the southern Gulf of Mexico and Florida Straits between 2005 and 2007 on commercial pelagic longline gear (n = 18) and recreational rod-and-reel gear (n = 1). The data from three tags indicated mortality events and were excluded from subsequent analyses. All PSATs were programmed to collect pressure (depth), temperature, and light-level data for 10 d at approximately 90-s intervals. These transmitted point data subsequently allowed the reconstruction of vertical movement patterns. The remaining 16 PSAT data sets indicate that sailfish are primarily associated with the upper surface waters within the top 20 m (75.7% of total time during the day versus 46.7% at night) but undertake numerous short-duration vertical movements below the local mixed layer to depths of 50–150 m, presumably to feed. Analyses of 2,279 individual vertical movements among all 16 tagged sailfish indicated two distinct types (short-duration “V” and longer-duration “U” movements) similar to those reported for white marlin Kajikia albida. Sailfish also exhibited movement type differences between diel periods (having higher proportions of V movements in daytime), suggesting directed foraging at depth. Although short-duration movement to depths by these tagged fish contribute a small percentage of the total time at depth, these depths overlap with the monitored shallow-set pelagic longline gear depths actively targeting swordfish by the vessel in the local fishery. These results suggest that time-at-depth histograms alone may be insufficient to capture feeding motivations at depth and, therefore, true interaction potentials between individual sailfish and pelagic longline gear. The sailfish Istiophorus platypterus is a large, cosmopoli- tan teleost found worldwide in tropical and subtropical waters, generally with higher concentrations near continental shelf areas (Nakamura 1985). Conventional tagging data have shown broad movements of sailfish within the western Atlantic Ocean (Ortiz et al. 2003), although no trans-Atlantic or trans-Equatorial movements have been documented (Orbesen et al. 2009). The latest assessment of the western Atlantic sailfish stock suggests that the stock is overfished and that this overfishing is primar- Subject editor: Michelle Heupel, James Cook University, Queensland, Australia *Corresponding author: kerstett@nova.edu Received August 14, 2010; accepted April 24, 2011 ily the result of international pelagic longline fleets targeting swordfish and tunas (SCRS 2009). In Florida, sailfish support a large, mostly catch-and-release recreational fishery based primarily in the coastal shelf re- gion between Key West and Jupiter (Jolley 1977). The Florida Straits have been closed to the U.S. pelagic longline fishery since 2001, primarily to protect local concentrations of juvenile swordfish Xiphias gladius. However, vessels continuing to use pelagic longline gear to the west and north of this closed area 353 354 KERSTETTER ET AL. occasionally encounter high rates of sailfish bycatch while targeting large swordfish and yellowfin tuna Thunnus albacares. Several studies using electronic tag technologies have shown that sailfish are capable of daily horizontal movements (e.g., Hoolihan and Luo 2007; Orbesen et al. 2008) on the scale of tens of kilometers. These ranges of distances would provide connectivity for sailfish between the portions of the Florida Straits that are open and closed to the pelagic longline fishery, a continuing source of domestic fisheries conflict in south Florida waters. Evaluating vertical habitat use by large pelagic fishes has his- torically presented challenges owing to a combination of their individual size, movement speed, and depth ranges. Previous work generally focused on the manual tracking of animals with acoustic tag technology for short periods of time with dedicated chase vessels (e.g., Jolley and Irby 1979). However, the development of pop-up satellite archival tag (PSAT) technology has enabled researchers to record environmental data on animals for much longer periods and at much more detailed resolution while eliminating the need for direct monitoring of the animal or fisheries-dependent returns of the tag. Electronic monitoring technology, such as small temperature and depth recorders (TDRs), has enabled a concurrent increase in our understanding of fishing gear behavior, including movements and effective fishing depths. The combined use of these technologies to describe both vertical short-duration movements and overall habitat utilization can provide insights into the vulnerability of bycatch species to various fishing gears, and allow for more-informed management measures. The present study used the point data from 16 PSATs with 90-s sampling period resolution for 10-d deployment durations attached to sailfish to describe the short- duration behavior and vertical habitat utilization of this species in the southern Gulf of Mexico and Florida Straits. METHODS Sailfish tagging occurred in two locations within the southern Gulf of Mexico: location 1, approximately 90 km south- southwest of Key West, Florida, in an area traditionally fished by the U.S. coastal pelagic longline fleet; and location 2, offshore of the island of Isla Mujeres, Mexico, the site of a large recreational fishery for sailfish (Figure 1). Tagging operations off Key West occurred aboard the U.S. commercial pelagic longline fishing vessel FV Kristin Lee during May 2006 and June 2007. The target species for all three trips was nominally swordfish, and (as is standard in the fishery) all sets were made overnight, gear deployment occurring at dusk and retrieval at dawn. The gear configuration was similar to that used throughout this local fishery and consisted of 18.3-m (10-fathom) leaders and 18.3-m (10-fathom) buoy float line lengths during each set in five-hook FIGURE 1. Southern Gulf of Mexico and Florida Straits study area in which deployments of pop-up satellite archival tags on sailfish were made. Filled dots indicate tagging locations, while open dots indicate the locations of first satellite transmission. Depth contours are shown for 200, 1,000, 2,000, and 3,000 m. SAILFISH HABITAT UTILIZATION AND VERTICAL MOVEMENTS 355 baskets (hooks between floats). Per current U.S. fisheries reg- ulations, all sailfish were caught on either non-offset size 16/0 or 10 ◦ offset size 18/0 circle hooks using squid Illex spp. or At- lantic mackerel Scomber scombrus bait. The tag deployment for the Isla Mujeres sailfish occurred aboard the sportfishing vessel Sea-D during May 2006 while trolling a ballyhoo baited with a non-offset size 7/0 circle hook. We used the Microwave Telemetry (Columbia, Maryland) Model PTT-100 HR satellite tag in all tag deployments during this study. Tags were rigged with approximately 16 cm of 136-kg (300-lb) test strength Momoi brand (Momoi Fishing, Ako City, Japan) fluorocarbon monofilament attached to a large hydro- scopic nylon intramuscular tag head with aluminum crimps per Graves et al. (2002). On all tags but the Isla Mujeres deployment, a 68-kg (150-lb) test strength Sampo brand (Sampo, Barn- eveld, New York) ball bearing swivel was incorporated midway along the tether to reduce twisting torque at the attachment location caused by drag forces on the tag. Tags sampled temperature, pressure (depth), and irradiance (light level) at 93-s intervals. This tag model also included emergency release soft- ware that automatically detached the tag if the pressure sensor indicated depths approaching the crush limit of the tag casing (ca. 2,000 m). All tags were preprogrammed to release from the fish after 10 d at large. Data were transmitted through the Argos satellite system while the tags floated at the surface following detachment from the animal. Tags used in this study transmitted archived data in “packets,” each encompassing several minutes of consecutive data points. However, each packet was transmitted in a discontinuous overall pattern such that gaps exist between packets within the transmitted record. This tag model also contained proprietary “SiV” programming, which directs the tag to only transmit data when an Argos satellite is expected to be above the horizon. This programming extends the onboard battery power and allows for additional Argos transmissions, thereby increas- ing the total transmitted data. To delineate the maximum effective fishing depths for the configuration of pelagic longline gear used by the commercial vessel, small TDRs (Model LTD-1100; Lotek Wireless, St. John’s, Newfoundland) were attached to the lower end of the middle branch lines (hook three in the five-hook baskets) during gear deployments (see additional details on placement in Ker- stetter and Graves 2006a). This model of TDR records pressure (as pounds per square inch [PSI]) and temperature at 14-s intervals. The pressure data from the TDRs were standardized from PSI to depth (m) with latitude and seawater density corrections using Harris (2000). Data from these TDRs were also used to confirm local mixed-layer depths (MLDs). Sailfish tagging.—Prior to deployment, all PSATs were allowed to cycle through the full internal activation process. The captain of the pelagic longline vessel identified incoming sailfish on the line, and individuals were initially evaluated as live or dead based on movement (or lack thereof) alongside the vessel. The sailfish tagged from the recreational vessel was identified to species prior to becoming hooked on one of the surface baits. Live fish were manually brought alongside the vessel rail and held briefly by the leader until calm. The PSAT tagging procedures used were identical to the ones described in Ker- stetter and Graves (2006b), although a shorter applicator tip (8 cm) was employed to compensate for the much more later- ally compressed sailfish body form. The nylon anchor attached to the PSAT tether was carefully inserted about 5–10 cm below the midpoint of the first dorsal fin to a depth of about 4–6 cm. This location on the fish provides an opportunity for the nylon tag head to pass through the dorsal pterygiophores with- out approaching the coelemic cavity (see Prince et al. 2002). A conventional National Oceanic and Atmospheric Administra- tion Fisheries Service Cooperative Tagging Center streamer tag was also attached posterior of the PSAT on all fish tagged from the pelagic longline vessel. Sailfish were released as soon as possible after tagging by cutting the leader near the hook unless the hook was readily accessible for manual removal. For the single recreationally caught fish, total time from capture to release was less than 10 min. No animals were resuscitated by either vessel platform after tagging. Prior to release, the hooking location (following the terminology of Yamaguchi 1989) and overall physical condition of the animal were noted, and fish lengths and weights were estimated. All other pertinent data—including the time of day, vessel location, and sea surface water temperature—were recorded immediately after tagging. Data analysis.—The net movement of tagged sailfish was estimated as the minimum straight-line distance (MSLD) traveled between the initial tagging location and the location of the first reliable satellite contact with the detached tag (inferred as the location of tag pop-up) using Argos location codes 1, 2, or 3 (position uncertainty, ≤1.5 km; CLS 2011) for the first or second day of transmission. “Great Circle” distances between these points were calculated with program inverse (version 2.0; NGS 1975; modified by M. Ortiz, National Marine Fisheries Service Southeast Fisheries Science Center, Miami, Florida). For analysis of diel differences, data were separated into day and nighttime periods. Sunrise and sunset times for approximated positions were obtained from the U.S. Naval Observatory (http://aa.usno.navy.mil). Because individual daily positions could not be matched with cloud cover data, no attempts were made to standardize light levels for local atmospheric conditions. Crepuscular periods were identified and excluded for diel analyses by removing the 30-min period before and after estimated times of local sunrise and sunset (corroborated with light-level data). Using only day and night period data, histograms were generated at 10-m (depth) and 1 ◦ C (temperature) intervals for each individual sailfish and compared using paired t-tests. Finally, depth differences between sequential ca. 90-s period point data were used to examine the range and speed of vertical movements. Due to the “packet” transmission of the archived tag data, the final data 356 KERSTETTER ET AL. sets occasionally had discontinuous intervals, usually less than 1 h in length. All discontinuous intervals within each tag record were identified and excluded from the individual dive analyses. Sea surface temperature (SST) was calculated as the average temperature for all depths 0–5 m to reduce fine-scale variability between measured data. Relationships between vertical habitat utilization and thermal structure of the water column used two calculated values for each photoperiod: SST and the mixed-layer depth temperature (MLDT = SST – 0.5 ◦ C as per Levitus 1982). Paired t-tests between diel periods were used to assess habitat utilization above and below respective SST and MLDT values for each 24-h period of the deployment. The structure of the transmitted data from the PSATs allows for a re-creation of the thermal environment surround- ing individual sailfish by using the fish as autonomous sam- plers of the water column (Boehlert et al. 2001; Block et al. 2003; Horodysky et al. 2007). Forty-eight hour periods were selected from three sailfish that had their tags physically recovered (hence, 100% data recovery) and demonstrated representative short-duration vertical movements to depth. Within a given 48-h period, archived temperature and depth data were used to create 96 temperature–depth profiles for each 30-min block of time. Temperature readings between data points were interpo- lated from these profiles at 5 m and 0.1 ◦ C resolution (MATLAB R2006a, version 7.2.0.232). To provide a visual description of local subsurface temperature and short-duration movements, in- terpolated temperatures and depth tracks were then superim- posed using the archived depth and temperature data recorded during the vertical movements of these individual fish per the methods of Horodysky et al. (2007). The structure of the PSAT data also allowed the reconstruction of individual vertical movements to depth. As all movements began and ended in shallow depths, these movements are referred to hereafter as “dives.” The characteristics of these individual dive events were assessed through a variety of analyses. Data from onboard vessel electronics, deployed TDRs (for the pelagic longline vessel sets), and reconstructed vertical profiles from the tag data sets all indicated an MLD of approximately 10 m in the waters of the southern Gulf of Mexico. A vertical movement was therefore considered a single dive if it (1) began at a depth less than 10 m, (2) incurred a maximum depth greater than 10 m, and (3) returned to a depth less than 10 m. Any vertical movement not meeting these three criteria or that was missing any data from within the movement itself was considered an “incomplete” dive event and excluded from subsequent analyses. Individual dives were then analyzed for maximum depth, minimum temperature, SST at beginning of dive, overall duration of dive, and the “interdive interval” (the period between the end of one dive event and the start of the next). Most dive events demonstrated a period of rapid movement to depth, fol- lowed by a relatively stable period at this depth before returning to near-surface waters. Any period of time at depth within an individual dive was termed “bottom time” and calculated as the period within the dive when the vertical movement rate was less than 5 m/min. All vertical movements and movement parame- ters were assessed through a manual review of each tag data set. Any extreme dive events were confirmed by corroboration with concurrent temperature data. Once a movement was classified as a dive event, subsequent tests were conducted comparing mean maximum depth and duration between diel periods among tagged individuals. Rela- tionships between mean dive depth and duration for pooled data were also explored through regression analyses as well as between diel periods. Significance was assessed at the α = 0.05 level. Dive characterization.—All 16 sailfish appeared to exhibit the two different types of dives described by Horodysky et al. (2007) for white marlin Kajikia albida. So-called “V-shaped” dives involved rapid descents with relatively small amount of time at depth or bottom time (≤10 min), and a rapid ascent to a shallower depth. Conversely, the “U-shaped” dives had similar rapid descents but a relatively longer time at depth (16–245 min) before the rapid ascent to shallower depths. Since the pri- mary difference between the dive types is amount of time spent at the lower depths of the dive, dive type can be determined as a function of bottom time. To confirm dive classification by bottom time, multivariate statistical techniques were applied to six different dive characteristics manually recorded for each completely transmitted dive of each surviving sailfish to determine if there were indeed two different dive types present, and what minimum and maximum bottom times best characterized a dive type. The six dive characteristic variables (dive duration, maximum depth, change in temperature, depth divided by dive duration, interdive interval, and bottom time) were entered into the quantitative techniques described by Lesage et al. (1999) and Horodysky et al. (2007). Dive characteristics were standardized (PROC STANDARD, SAS version 9.2; SAS Institute, Cary, North Carolina), and a principal components analysis (PCA) was used to both elimi- nate collinearity and produce a smaller set of orthogonal factors to input into cluster analysis (Horodysky et al. 2007). Four orthogonal factors were derived from the PCA (dive duration, maximum depth, change in temperature, and interdive interval) and were entered into a hierarchical complete-linkage cluster procedure to ascertain the appropriate number of clusters and dive types, and to determine seed points for nonhierarchical K-means clustering (Horodysky et al. 2007). Hierarchical complete-linkage clustering is an agglomerative method which classifies clusters by the maximum distance between one cluster and the next (Hair et al. 1998). The number of dive types sufficient to capture the variability between dives was determined by examining the agglomerative coefficient, the squared Euclidean distance between two clusters being combined, from 2 up to 10 clusters (Horodysky et al. 2007). The cluster centroids that resulted from the complete-linkage clustering are next entered into a nonhierarchical K-means clustering that further fine-tuned the formed clusters. Observations were assigned to the cluster with the centroids with the closest Euclidean distance, and new SAILFISH HABITAT UTILIZATION AND VERTICAL MOVEMENTS 357 TABLE 1. Summary of satellite archival tagging deployments for sailfish in the southern Gulf of Mexico. The ACESS score refers to a physical condition index based on a 10-point scale (10 being the highest score; see Kerstetter et al. 2002 for further details); MSLD = minimum straight-line distance traveled. The three mortalities described within text are not included. Sailfish Date deployed Hooking location Hook size Hook removed ACESS score Estimated length (cm) Reporting (%) MSLD (km) 6-01 3 May 2006 Corner 16/0 Yes 9 137 59 448.0 6-02 4 May 2006 Lower jaw 18/0 Yes 9 183 82 375.5 6-03 4 May 2006 Fouled 16/0 No 8 168 63 150.1 6-04 4 May 2006 Isthmus 18/0 Yes 10 183 55 188.6 6-05 4 May 2006 Corner 16/0 No 10 168 68 332.4 6-06 5 May 2006 Eye socket 16/0 No 9 152 75 554.9 6-07 5 May 2006 Fouled 16/0 Yes 8 152 65 a 97.3 6-08 5 May 2006 Lower jaw 18/0 Yes 8 168 40 193.5 6-09 6 May 2006 Corner 18/0 Yes 8 152 68 447.0 6-10 (IM) b 31 May 2006 Corner 7/0 Yes 10 137 c 49 217.1 7-01 6 Jun 2007 Corner 16/0 No 9 122 75 522.9 7-03 6 Jun 2007 Corner 16/0 No 6 122 87 125.0 7-04 6 Jun 2007 Corner 16/0 No 10 122 74 406.2 7-05 6 Jun 2007 Corner 16/0 No 10 137 86 564.0 7-06 9 Jun 2007 Corner 16/0 No 5 107 88 717.3 7-07 9 Jun 2007 Corner 16/0 No 6 122 88 67.4 a Original reporting percentage; tags were later returned, allowing a 100% data recovery rate. b Animal tagged off the recreational vessel in Isla Mujeres, Mexico. c Not estimated. centroids were calculated after each iteration until the changes in centroids become small or zero (Horodysky et al. 2007). Dive classification was confirmed by discriminant function analyses, using the two nearest neighbors to identify which cluster (dive type) to be assigned (Lesage et al. 1999). Percentages of misclassified dives, or error rates, were calculated by cross-validation. A matrix of minimum bottom time values (1, 5, 10, and 15 min) for U-shaped dives was compared with a maximum bottom time values for V-shaped dives to investigate which combination of minimum and maximum bottom time best rep- resented dive type. The resultant dive types were then entered into the quantitative methods described previously to determine which minimum and maximum values were agreed upon by both dive type via bottom time and objectively by multivariate statistical techniques. The set of minimum and maximum values that covered the broadest scope of dives and had the lowest percentages of misclassified dives was used to determine dive type. RESULTS Tagging Events Eighteen PSATs were deployed on sailfish caught on pelagic longline gear targeting swordfish in the southern Gulf of Mexico between November 2005 and July 2007. Overall bycatch of istiophorid billfishes comprised less than 3% by number of the total catch on the three observed trips. One PSAT was deployed on a sailfish caught from a sportfishing vessel off Mexico in May 2006. Three sailfish caught on pelagic longline gear died shortly after release, and the data from these fish were excluded from subsequent analyses (see Kerstetter and Graves 2008). A summary of tagging information and the physical condition of the surviving tagged animals is presented in Table 1. For all 16 PSATs, an average of 70.3% (range = 40–88%) of the archived data were successfully recovered through the Argos system. Four archival data sets (2006: n = 1; 2007: n = 3) were recovered after the tags washed up onto Atlantic beaches and were returned to the authors. All (100%) of the archived data were recovered from these four returned tags and included in subsequent analyses. Horizontal Movement Individual sailfish moved away from the tagging location various distances and and in various directions (mean distance = 337.9 km; range = 97.3–564.0 km). There was no relationship between MSLD traveled and estimated individual size. Three of the fish tagged within the U.S. Exclusive Economic Zone (EEZ) crossed into foreign EEZ waters, including the Bahamas (n = 2) and Cuba (n = 1), while the fish tagged in Mexican waters remained within the Mexico EEZ (Figure 1). Depth and Temperature There were no significant diel differences in either the time-at-temperature or time-at-depth distributions between the 2 years of this study, and data were subsequently pooled to in- clude fish from both years. Sailfish demonstrated a very strong 358 KERSTETTER ET AL. FIGURE 2. (A) Combined time-at-depth and (B) time-at-temperature histograms for 16 sailfish tagged with pop-up satellite archival tags for 10-d deployment durations in the southern Gulf of Mexico and Florida Straits, 2006 and 2007. Error bars indicate SEs around mean values. association with warm surface waters (Figure 2A, B), spend- ing approximately 34% (SD, 13.2) of their total time in the upper 10 m of the water column and 41% (SD, 10.7) within the 10–20-m stratum. Sailfish spent 12.4% (SD, 12.9) of their time at depths ranging from 20 to 50 m, and only 10.6% (SD, 26.7) at depths greater than 50 m. Broad standard errors reflect large within-individual (daily) variation in time at depth rather than differences among individuals. The absolute depth difference between sequential 90-s point measurements (“delta D”) observed in three of the fish with 100% data recovery found a highly significant difference between day and night periods (t = –4.58, P  0.001 using Satterthwaite test for unequal SAILFISH HABITAT UTILIZATION AND VERTICAL MOVEMENTS 359 FIGURE 3. (A)–(C) Detailed 48-h pattern of vertical movements overlaid on re-created local temperature-at-depth profiles generated from archived tag data for three sailfish tagged with pop-up satellite archival tags for 10-d deployment durations in the southern Gulf of Mexico and Florida Straits, 2006 and 2007. Clear diel differences in dive periodicity are evident in (A) but not (B), and a moderate effect of diel period is displayed in (C). The black bars along the top of each panel represent the local periods of night generated from archived light-level data from the tags. Night periods varied slightly in length between fish owing to different deployment dates. variances), with sailfish moving vertically much more frequently between depths at night. Pooled temperature data demonstrated that sailfish spent 89.6% (SD, 45.4) of their time in water temperatures ranging from 25–29 ◦ C (Figure 2B), although archived SSTs occasionally reached over 30 ◦ C. Many individuals exhibited considerable daily variation in the temperature–depth data over the course of the 10-d tag deployment period (Figure 3, A–C), including deep short-duration movements below the MLDT. The absolute temperature difference within each dive event (“delta T”) showed that 71.7% (SD, 29.7) occurred between 0 ◦ C and 2.0 ◦ C, 99.2% (SD, 6.4) occurring between 0 ◦ C and 8.0 ◦ C (Figure 4). All of the fish in this study spent more time at depths below the MLD during daylight hours (significantly for 14 fish of the 16 total; P < 0.05; Table 2). Individual fish exhibited different patterns regarding total time spent below the MLD; however, of the four individuals showing a significant difference between day and night periods for time below the MLD, three were at those depths more at night and one during day. Pooling all individual sailfish, a regression analysis of time spent below the MLD and individual body size (as estimated LJFL) showed no significant effect (adjusted r 2 = 0.116, F = 2.9, P = 0.1069). A total of 2,279 complete individual dive events were ex- amined. To minimize autocorrelation effects between individual dives, a mean maximum dive depth and mean dive duration were 360 KERSTETTER ET AL. FIGURE 4. Percentages of dives versus differences between the local sea surface temperature (SST) and the minimum temperature encountered on a dive event by 1 ◦ C intervals for 16 sailfish tagged with pop-up satellite archival tags for 10-d deployment durations in the southern Gulf of Mexico and Florida Straits, 2006 and 2007. calculated for day and night for each fish. Night dive events had a mean maximum depth of 38.6 m and mean dive duration of 19.4 min, while day dive events had a mean maximum depth of 45.0 m and a mean duration of 14.4 min. Relationships between mean dive depth and mean dive duration within each diel period were significant (night: adjusted r 2 = 0.615, P < 0.001; day: adjusted r 2 = 0.746, P < 0.001), although the regressions were not significantly different from each other (Fisher’s z comparison: z =−0.615, P > 0.25; Cohen and Cohen 1983). Comparisons of dive depth and duration by diel period for all TABLE 2. Comparisons between day and night periods for the percentage of time spent below the mixed-layer depth (MLD) and the percentage of time spent below the sea surface temperature depth (SSTD). Asterisks denote significant differences. Percent < SSTD Percent < MLD Sailfish number Mean day Mean night Significance Mean day Mean night Significance 6-01 0.051 0.583 * 0.913 0.179 * 6-02 0.267 0.020 0.625 0.369 * 6-03 0.121 0.117 0.814 0.642 * 6-04 0.257 0.239 0.784 0.556 * 6-05 0.219 0.133 0.318 0.745 * 6-06 0.285 0.219 0.929 0.650 * 6-07 0.328 0.342 0.508 0.164 * 6-08 0.184 0.187 0.800 0.673 * 6-09 0.235 0.412 0.690 0.167 * 6-10 (IM) a 0.056 0.112 0.919 0.793 * 7-01 0.345 0.277 0.606 0.483 7-03 0.211 0.296 0.675 0.200 * 7-04 0.035 0.061 * 0.766 0.473 * 7-05 0.082 0.278 * 0.900 0.493 * 7-06 0.400 0.097 * 0.319 0.293 7-07 0.238 0.361 0.533 0.328 * Mean 0.207 0.233 0.694 0.450 * a Animal tagged off the recreational vessel in Isla Mujeres, Mexico. SAILFISH HABITAT UTILIZATION AND VERTICAL MOVEMENTS 361 0 5 10 15 20 25 30 35 40 0 20 40 60 80 100 120 140 160 180 200 Depth (m) Percentage of Total Time Shallow-Set Pelagic Longline Sailfish FIGURE 5. Combined time-at-depth histogram for hook depths and sailfish depth utilization during the same period as pop-up satellite tag deployments in the southern Gulf of Mexico, 2006. Pelagic longline gear hook depths are for the deepest (middle) hook within a five-hook basket of shallow-set pelagic longline gear representative of commercial gear deployments in the area. Depth distributions represent combined day and night periods. Error bars are omitted for clarity. pooled individual dives also resulted in significant relationships (night: adjusted r 2 = 0.615, P < 0.001; day: adjusted r 2 = 0.746, P < 0.001), although the regressions for the day and night diel periods were not significantly different from each other (Fisher’s z comparison: z = –0.615, P > 0.25). Depths of Shallow-Set Pelagic Longline Gear Thirty-one individual TDR deployments were conducted during four sets in 2006, all in hook position 3 of the five-hook baskets, the middle and presumably deepest hook position. The mean depth of the TDRs was 42.3 m (±SD 19.6), and the maximum depth recorded was 143.8 m. The time at depth distribution for the pooled TDR data set for the deepest hook position and the combined day and night time depth distribution of sailfish tagged in 2006 is presented in Figure 5 and shows a large percentage of overlapping depths (including the implied greater percentages for the pelagic longline gear at the two shallower hook positions). However, these apparently overlapping depth distributions do not reflect the actual movements of individual sailfish to greater depths, even if such movements are for relatively minor proportions of total time at depth. Examinations of the sequential data “tracks” for these sailfish (e.g., Figure 3A–C) showed frequent short-duration movements below the depths of the shallow-set pelagic longline gear. Dive Characterization Appropriate bottom time limits to determine dive type were considered by comparing differing minimum bottom time values for U-shaped dives and maximum bottom times for V-shaped dives for each minimum and maximum value (i.e., a V-shape maximum of 1 min would be compared for the entire series of U-shape minimums [1, 5, 10, and 15 min]). A maximum bottom time of 10 min for V-shaped dives and a minimum bottom time of 15 min for U-shaped dives yielded the highest percentage of total dives (91.5%) with the lowest percentage of misclassified dives (5.7%); 224 dives types remain undetermined with bottom times between 11 and 14 min. Cluster analysis showed that after the joining of two clusters the agglomerative coefficient dropped precipitously (agglomerative coefficient of 2.5 at two clusters, dropping to 0.6 at three clusters), implying that two dive classifications is sufficient. Differences were observed between the dive characteristics of U-shaped versus V-shaped dives. U-shaped dives had deeper mean dive depths (53.5 ± 34.0 m), longer dive durations (28.0 ± 27.1 min), and a larger change in temperature (5.0 ± 12.5 ◦ C) than V-shaped dives (38.2 ± 26.2 m versus 10.2 ± 9.5 min, 2.2 ± 5.6 ◦ C). However, U-shaped dives and V-shaped dives had similar interdive intervals (24.4 ± 50.2 min and 24.8 ± 66.8 min, respectively). DISCUSSION The description of sailfish behavior is of interest not only to the various fishing constituencies but also to those seek- ing gear-based bycatch avoidance solutions and habitat-based standardization methods for stock assessment purposes. Using point-level data allowed for a clearer characterization of short- duration sailfish movements, as opposed to overall habitat utilization through summary histograms. The ability to recreate individual dives over a relatively longer period of time than previous acoustic studies also presented a better picture of sailfish behavior, including the potential for interactions of individuals with pelagic longline gear. Furthermore, the successful deployment of these PSATs with no premature releases on sailfish supports the observation that fishes smaller than large marlin and tunas can accommodate these tags for short-duration deployments (Kerstetter and Graves 2006b; Horodysky et al. 2007). The development of smaller PSAT models will clearly expand the size range, and thus species list, of similar tagging studies in the future. Horizontal Displacement The horizontal movements of sailfish in multiple directions away from the initial tagging locations for long distances were similar to the behavior reported by Graves et al. (2002) for blue marlin Makaira nigricans, Kerstetter and Graves (2006b) for white marlin, and Sippel et al. (2007) for striped marlin K. au- dax, as well as that seen for sailfish by Prince et al. (2006) and Hoolihan and Luo (2007). Only two of the 16 tagged sailfish had MSLDs of less than 100 km over the 10-d deployment period. The horizontal displacements observed in this study may be re- lated to spawning, as they are consistent with the postspawning movements northward along the shelf edge described in Jolley and Irby (1979). The spawning period for sailfish in the southern Gulf of Mexico and Florida Straits is from late April through June (Voss 1953), and female sailfish caught in areas southwest of the Florida Keys during this time often have hydrated [...]... preferences in the Arabian Gulf (Hoolihan and Luo 2007) showed only rare utilization of depths greater than 50 m The present study found a broad range of depths encountered by these tagged individuals, even if the majority of the time as a whole was spent within the upper 20 m However, the range of possible depths for the sailfish tagged in the southern Gulf of Mexico far exceeded those for the animals in the. .. Jolley (1977) used detailed analyses of recreational catch records to argue that sailfish indeed undertake a seasonal movement southward into the Florida Keys during the winter and spring, then moving northward through the summer The movement of the majority of the tagged animals in this study northward along the Gulf of Mexico and Atlantic edges of the shelf supports the patterns observed by Jolley (1977)... but the tracks were very short in duration The comparatively shallow waters of the Arabian Gulf also limited the range of temperatures available to be encountered by the sailfish described there, although no such limitation exists in the southern Gulf of Mexico and Florida Straits The so-called “thermal inertia” hypothesis suggests that large-bodied fishes retain heat and more effectively forage in colder... tagging Gulf and Caribbean Research 20:97–102 Kraus, R T., and J R Rooker 2007 Patterns of vertical habitat use by Atlantic blue marlin (Makaira nigricans) in the Gulf of Mexico Gulf and Caribbean Research 19:89–97 Lesage, V., M O Hammill, and K M Kovacs 1999 Functional classification of harbor seal dives using depth profiles, swimming velocity, and an index of foraging success Canadian Journal of Zoology... rates in the central Pacific were affected by overall gear depth and the position of the depth relative to the local thermocline More recent efforts by Bigelow et al (2006) and Rice et al (2007) to describe the depths of the pelagic longline gear have begun to clarify some of the differences between the predicted depths of Yoshihara (1954) and the actual depths actively fished by the gear However, the. .. determined by ultrasonic and pop-up satellite tagging Marine Biology 146:1015–1029 Hoolihan, J P., and J Luo 2007 Determining summer residence status and vertical habitat use of sailfish (Istiophorus platypterus) in the Arabian Gulf ICES Journal of Marine Science 64:1–9 Horodysky, A Z., R Latour, D W Kerstetter, and J E Graves 2007 Habitat preferences and vertical movements of white marlin (Tetrapturus albidus)... are very consistent with the Brill and Lutcavage (2001) hypothesis The dive events in these 16 tagged sailfish showed similarities with the V- and U-shaped patterns described in Horodysky et al (2007) for white marlin, and although there was a continuum of bottom times, further analyses of the “bottom time” factor confirmed that there are two main types of individual dive events These results suggest that... Prince, J E Serafy, D B Holts, K B Davy, J G Pepperell, M B Lowry, and J C Holdsworth 2003 Global overview of the major SAILFISH HABITAT UTILIZATION AND VERTICAL MOVEMENTS constituent-based billfish tagging programs and their results since 1954 Marine and Freshwater Research 54:489–508 Pepperell, J G and T L O Davis 1999 Post-release behavior of black marlin (Makaira indica) caught and released using... statistics using physiological, ecological, or behavioral constraints and environmental data, with an application to blue marlin (Makaira nigricans) catch and effort data from the Japanese longline fisheries in the Pacific Bulletin of the InterAmerican Tropical Tuna Commission 21:171–200 Hoolihan, J P 2005 Horizontal and vertical movements of sailfish (Istiophorus platypterus) in the Arabian Gulf, determined... horizontal behavior of sailfish is likely mediated by many different factors, including prey density, local oceanographic conditions, and perhaps even spawning events and seasonal effects as previously discussed The varying behavior between the southeastern Gulf of Mexico sailfish in the present study and those described in Hoolihan (2005) from the Arabian Gulf may simply reflect these differences and preclude . and vertical habitat utilization of this species in the southern Gulf of Mexico and Florida Straits. METHODS Sailfish tagging occurred in two locations within the southern Gulf of Mexico: location. funders in the common goal of maximizing access to critical research. Sailfish Habitat Utilization and Vertical Movements in the Southern Gulf of Mexico and Florida Straits Author(s): David W online DOI: 10.1080/19425120.2011.623990 ARTICLE Sailfish Habitat Utilization and Vertical Movements in the Southern Gulf of Mexico and Florida Straits David W. Kerstetter,* Shannon M. Bayse, and

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